1. Field of the Invention
This invention relates generally to a system and method for introducing true time delays in an RF signal which is applied to radiating elements of a phased array antenna, and more particularly to an active recirculating delay line for introducing true time delays in pulsed RF signals being delivered to the radiating elements of a phased array antenna.
2. Description of Related Art
In the field of radar, systems have been developed that use antennas in which the transmitted power is divided among many radiating elements and in which the phase of each element can be dynamically varied. In such a phased array antenna, the beam can be steered by appropriately varying the phase of the radiating elements. Consequently, antenna beam steering can be accomplished without being constrained by mechanical limitations, such as the rotation of the antenna.
Minimum side lobe level and accurate beam pointing of the phased array antennas require that the actual phase and amplitude distribution of the electromagnetic field generated over the antenna aperture has a minimum ripple, meaning the generated signal approaches the desired smooth, continuous theoretical electromagnetic field distribution as closely as possible. The fact that there are a large, but finite, number of array elements results in a certain minimum amplitude and phase ripple in the electromagnetic field over the antenna aperture. This ripple determines the actual side lobe level and accuracy of the antenna beam pointing.
Any deviation from the minimum desired phase and amplitude distributions reduce the accuracy of beam pointing and increase the side lobe levels of the phased array antenna.
Of those phased array antennas currently in use, most are in fact reduced phase shifter arrays, in which the maximum phase shift that a phase shift element needs to provide is 360.degree., which is equivalent to a delay length of one wavelength. If delay lines differ in lengths by one or more multiples of the wave length, the continuous wave (CW) signals produced would be indistinguishable. Thus, for CW phased array systems, a maximum delay line length of one wavelength, which introduces a phase shift of 360.degree., is sufficient. When dealing with RF pulsed signals, however, processing these signals in reduced phase shifter phase array antennas cause the signals to suffer from pulse stretching and deterioration of the rise and fall times of the pulsed signal. More importantly, higher side lobe levels result. High side lobe levels are very undesirable in radar because they permit higher levels of unwanted signals to be picked up by the antenna system. For reasons including high RF losses, high cost and size and weight considerations, a true time delay for a phased array antenna of any practical significance has yet to be constructed. It would therefore be advantageous to provide for a true time delay for a phased array antenna which can delay the signals without degenerating the pulsed signal.